A conventional printed circuit device, such as a flexcable package or printed circuit board, has pads for attachment of surface mounted devices and connection to other circuit elements. The surfaces are masked and the exposed pads prepared with appropriate coatings for the attachment process. Where multiple attachment techniques are used, the preparatory coatings of the pads must be adapted to the particular connective process employed.
Several types of attachment require pad coatings with differing physical characteristics. The application described in this document, as an example of a device employing multiple attachment techniques, is a flexcable that carries wirebond chips and discrete components in addition to interconnecting with the balance of the host device. The flexcable illustrated and described is a typical flexcable which is supported by a disk drive actuator rotor, connects to the transducer head lead wires and the host device electronics and supports an arm electronics (AE) module and discrete components. This type flexcable package possesses mounting and attach pads that support four distinct types of interconnection requirements:
1. Chip on flex (COF) attach pads for direct chip attach (DCA) aluminum wire wedge bonding of the AE chip.
2. Pads for solder attachment of passive discrete surface mount technology (SMT) components.
3. Pad surfaces for connector contacts of an SMT pad on pad connector required for card electronics to flexcable interconnection.
4. Head lead pads for wirebond attachment of gold plated copper wire used for magneto resistive (MR) transducer head to flexcable interconnections.
In this example of flexcable structure, reliable wirebonding of the AE module chip-to-flex and head lead to flex requires the use of soft, ultra high purity noble or semi-noble plated surface finishes such as gold, silver, or palladium over nickel based undercoatings. Pad on pad contact areas require hard, wear resistant surfaces usually provided through plating of finishes such as gold flashed nickel-palladium or cobalt/nickel hardened gold over nickel based underplatings. Further, any and all surface finishes must also be SMT compatible and provide both adequate solderability and favorable metallurgical interface reactions to ensure formation of consistently robust solder joints.
In many devices that have diverse interconnection types, surface finish materials selection problems can be circumvented by selectively plating given carrier regions with different metallurgies that support specific interconnect functions. Unfortunately, selective plating applications require circuit masking and associated mask application, stripping and cleaning processes. Therefore, when multiple regions of different platings are needed to support several interconnection functions, use of selective area plating can drive exceedingly complex, time consuming, and expensive circuit manufacture. As electronic devices become price sensitive commodity products, designs must have inherent low cost of manufacture to help enable competitive pricing. Further, the use of successive masking, plating and stripping sequences can result in the presence of masking residues which can complicate or compromise the connections that are implemented after the initial masking and plating sequence. Clearly, a cost effective universal interconnect finish has both strategic and economic value.
Numerous material sets have been proposed and used for terminal pad surface preparation, but these have been directed either to solder and wirebond applications or to connector applications, not both. In practice, it has been a requirement that a thin intermediate layer of harder material must be used in the soldering and wirebonding environment, while a thick intermediate layer must be present for mechanical connector applications.